US3791940A - Process for sealing anodized aluminum - Google Patents

Abstract

IMPROVING AN ANODIC OXIDE COATING ON ALUMINUM BY SEALING WITH AN AQUEOUS HYDROLYZABLE METALLIC SALT AND THEN TREATING WITH AT LEAST ONE AQUEOUS MINERAL ACID TO REMOVE SMUDGE FORMED DURING SEALING.

Description

3,791,940 Patented Feb. 12, 1974 United States Patent ()flice I 3,791,940 w PROCESS FOR SEALING ANODIZED ALUMINUM Andrew Alexander, Toledo, Ohio, assignor to Aluminum Company of America, Pittsburgh, Pa. No Drawing. Continuation of abandoned application Ser. 'No. 49,944, June 25, 1970. This application May 12,

1972, Ser. No. 252,768

' Int. Cl. C23c 7/00,: C231? 17/00 U8. (3]. 204-35 N 1 8 Claims ABSTRACT OF THE DISCLOSURE Improving an anodic oxidecoating on aluminum by sealing with an aqueous hydrolyzable metallic salt and then treating with at least one aqueous mineral acid to remove smudge formed during sealing.

BACKGROUND OF THE INVENTION This is a continuation of application Ser. No. 49,944, filed June 25, 1970, now abandoned.

This invention relates to improving an anodic oxide coating on an aluminum substrate. More particularly, it

relates to treatment of an apodic oxjdecoating which has not been organic dy e colored and is present on an aluminum substrate. Aluminum as used herein includes aluminum and aluminum'base alloys containing 50% or more by Weight aluminum.

It is known to seal anodized aluminum, that is, aluminum or aluminum'ba'se alloys having an anodic oxide coating thereon, with hot or boiling water. It is further known to improve hotwatersealing, of the anodic oxide SUMMARY on THE INYENTION In its broader aspects my invention involves treating an anodic oxide coating which has not been organic dye colored and is present on analuminum substrate with an aqueous hydrolyzable metallic salt so as to seal said anodic oxide coating with formation'thereon of a removable sealing smudge andthen treating said coating with at least one aqueous mineral acid so as to remove sealing smudge and leave said anodic oxide coating sealed. The anodic oxide coating treated according to the invention may be formed conventionally, for example, by using a sulfuric acid electrolyte or a 'sulfo'phthalic acid/sulfuric acid electrolyte. When the anodic oxide coating has a thickness of less than about 0.7 mil, it is preferred to use at least about 0.1 gramof hydrolyzable metallic salt per liter of water used in the sealing step. When the anodic oxide coating has a thickness of at least about 0.7 mil, it is preferred to use at'least about 1 gram of hydrolyzable metallic salt per liter of water. Representative of hydrolyzable metallic salt per liter of water. Representative of hydrolyzable metallic salts which maybe used in the seal ing step are nickel acetate, cobalt acetata'nickel sulfate, cobalt sulfate, 'the same and like 'or'similar salts of aluminuin, zinc, copper, lead iandof the alkali metals and the like. Preferred arelnickel acetate and cobaltacetate.

The acid treatment step" of the invention maybe performed by spraying, dipping or thelike. Illustrative mineral acids which may be used according to the invention in treating the sealed anodic oxide coating so as to remove sealing smudge therefrom include nitric, hydrochloric, sulfuric, phosphoric and chromic. Mixtures of the two or more of these acids are permissible according to the invention. Preferred acids are nitric and sulfuric. Acid concentrations as high as the solubility limit may be used according to the invention. In like manner, it is possible according to the invention to vary the temperature of the acid used as the need arises, that is, in such a way as to remove an optimum amount of smudge in a convenient time period. In many instances, by properly varying the concentration of the acid, the temperature, and also the time of acid treatment, as well :as the sealing conditions, for example, the concentration of salt used and time spent during sealing, it is possible according to the invention to produce a substantially smudge-free sealed anodic oxide coating. While this may be accomplished in some instances by employing a mineral acid treatment step as short as a few seconds, it may sometimes require several minutes or even somewhat longer.

According to my invention, if a substantially smudgefree surface does not result after a short time acid treatment step, such surface may be obtained by then wet wiping the sealed anodic oxide coating. According to a further embodiment of the invention, the anodic oxide coating from which sealing smudge has been removed by treating with acid is receptive to adherence of a lacquer, paint, enamel or like coating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples are illu strative of the invention.

Example 1 This example illustrates sealing and removal of smudge according to the present invention, including the advantage of not undoing bythe step which involves removal of smudge, the sealing accomplished by treatment with hydrolyzable metallic salt. The starting material used in this example was made up of two lots of specimens 3 x 6" x .064" of Aluminum Association Alloy No. 1100 clad with Aluminum Association Alloy No. 1100 having an anodic oxide coating thereon produced by conventional treatment with a sulfophthalic acid/ sulfuric acid electrolyte. The two lots were similarly treated except for use of nitric acid in the smudge removal step for one lot and use of hydrochloric acid for the smudge removal step of the other lot. All specimens were sealed for 15 minutes in 3 grams per liter (g./l.) nickel acetate at a temperature of 212 F. and a pH of 6. At this point all specimens exhibited a white, chalky smudge on the surface of the sealed anodic oxide coating. They were then dipped in an acid bath for 3 minutes. The concentration of the nitric acid used was 41 weight percent and of the hydrochloric acid used, 25 weight percent. The acids were at room temperature, that is, about 70 F. In all instances, after the treatment with acid, the specimens exhibited a substantially smudge-free sealed anodic coaitng. Efficiency of sealing was checked by an acid dissolution test in which weight loss upon acid dissolution was measured in milligrams per square inch (mg/in?) for the individual specimens after the smudge had been removed by the acid treatment step of the invention. The average weight loss for the samples treated with nitric acid was 0.47 mg./in. and for the samples treated with hydrochloric acid 0.28 mg./in. indicating retention of the sealed condition. In the acid dissolution test a weight loss of greater than 2 mg./in. would indicate-that the hydrolyzable metallic salt seal subsequent treatment step. By way of further example,

similar results are obtainable by treatment with chromic, phosphoric or sulfuric acid.

Example 2 This example illustrates that the concentration of the mineral acid used in the acid treatment step of the invention may be varied and yet effect the desired smudge removal. The sheets used for the specimens were 3" x 6" x .064" Aluminum Association Alloy No. 1100 clad with Aluminum Association Alloy No. 5657. In this example hydrochloric acid was the mineral acid used. All the specimens in this example were sealed for 15 minutes in 3 g./l. nickel acetate at a temperature of 212 F. and a pH of 6.2. After the sealing step the anodic oxide coating on all specimens was substantially covered with smudge. The treatment conditions for the acid treatment step are given in Table I, including the type and thickness of anodic oxide coating on the starting material and the condition of the specimens observed after the acid treatment. The specimens which were not smudge-free after the acid treatment were susceptible to being made so by wet wipmg.

Aluminum Association Alloy No. 1100 having an anodic oxide coating thereon, produced by treatment with a sulfuric acid electrolyte, were treated according to the procedure of the invention. Sealing was accomplished substantially as in Example 3 except that the pH of the nickel acetate solution was 6.2 instead of 6.0. Most of a chalky smudge visible after the sealing step was removed by the acid treatment step, which consisted of immersion of the samples in a solution of nitric acid of 41 weight percent at room temperature (about 70 F.) for 3 minutes, followed by rinsing in clean water and drying on the steam table. A small insignificant amount of bottomedge smudge susceptible to being easily wiped off remained. Two of the six specimens were subjected to the above-described weight loss by acid dissolution test. Weight losses of 0.27 and 0.40 mg./in. were obtained, demonstrating that the anodic oxide coatings remained well sealed after immersion in the nitric acid. The remaining four specimens were lacquered and then tested for adhesion of the lacquer. No specimen exhibited failure when subjected to the above-described adhesion test.

TABLE I Condition aiter H01 treatment of Condition alter Condition after about 1.25 mil H01 treatment of HCl treatment of Anodic oxide Weight about 0.95 mil Weight about 0.32 mil Weight HCl, coating formed in loss anodic oxide loss anodic oxide Loss wt. Temp., suliophthalic (mg/sq. coating formed in (mg/sq. coating formed in (mg/sq. Spec. No. percent F. acid/sulfuric acid in) sulfuric acid in.) sulfuric acid in.)

70 Smudge free 0. Smudge free 0. 54 Smudge tree 0.30 37. 6 70 -.-do. 0.35 (10. 0. 54 do. 0. 33

1 After about 1.5 minutes. 2 After about 2 minutes. a After about 3 minutes.

Example 3 Example 5 This example illustrates an embodiment of the invention in which, after the acid treatment step, the sealed anodic oxide coating is coated with lacquer which adheres thereto. Five sheets of Aluminum Association Alloy No. 1100 clad with Aluminum Association Alloy No. 1100, 4" x 10" x .064", having an anodic coating thereon formed by treatment in a sulfophthalic acid/sulfuric acid electrolyte were sealed for 15 minutes in a boiling nickel acetate aqueous solution containing 3 g./l. nickel acetate at a pH of 6. A white, chalky smudge developed upon most of the exposed surfaces. The specimens were then immersed for 3 minutes in a 41 weight percent nitric acid solution at room temperature (about 70 F.). Smudge was removed from substantially all of the surfaces except for small traces of smudge along the edges insuflicient to require rejection of the specimens for many applications. Two specimens were subjected to the above-mentioned acid dissolution test. Weight losses for these specimens were 0.27 and 0.40 mg./in. respectively, indicating wellsealed anodic oxide coatings. The other three specimens were coated with lacquer and then tested for lacquer adhesion. No failures occurred for these three specimens, that is, after coating with an acrylic lacquer and drying substantially no lacquer was removed by attaching a strip of adhesive tape and then pulling it oflf.

Example 4 This is an additional example illustrating smudge removal by acid treatment after hydrolyzable metallic salt sealing according to the invention. Six sheets 4" x 6" x .064 Aluminum Association Alloy No. 1100 clad with This example illustrates use of the preferred sulfuric and nitric acids in the acid treatment step of the invention. Eighty specimens of Aluminum Association Alloy No. 5657 having an anodic oxide coating thereon were treated according to the invention. The anodic oxide coating on all eighty specimens was produced by treatment in a sulfuric acid electrolyte. The anodic oxide coating as formed on forty specimens was approximately 0.3 mil, and 0.9 mil for the remaining forty. All specimens were sealed for 15 minutes in a 212 F. aqueous solution of nickel acetate (3 g./l.) at a pH of about 6.2. At this point all of the specimens exhibited an adherent, heavy white, chalky smudge.

After the sealing step twenty of the specimens which had an original anodic oxide coating of 0.3 mil and twenty of the specimens having an original anodic oxide coating of 0.9 mil were immersed for 1, 3, 5 and 10 minutes in 25, 41, 50 and 70 weight percent nitric acid at room temperature (about 70 F The remaining twenty specimens having an original anodic oxide coating of 0.3 mil and the remaining twenty specimens having an original anodic oxide coating of 0.9 mil were immersed after the sealing step for 1, 3, 5 and 10 minutes in 24 and 50 weight percent sulfuric acid at F. The condition of all of the sealed anodic oxide coatings after the acid treatment step was observed for all specimens. The sealed anodic oxide coatings on all of the specimens treated with the nitric acid in which the original anodic oxide coating thickness was 0.3 mil were substantially smudge free. Of the anodic oxide coatings of 0.9 mil which were treated with nitric acid, all were substantially smudge free after the acid treatment step in which the concentration of acid was atleast vweight percent and the treatment time at The anodic oxide coating on each panel was sealed by 'least.3.minutes., treatment for 15 minutes in aqueous nickel acetate (3 Thesealed anodic oxide. coatings on all twenty of the g./l.) at a pH of 5.9. Each panel was then immersed for specimens which had an original anodic oxide coating 3 minutes in 41 weight percent nitric acid at room temthickness of 0.3 mil, and were treated with sulfuruic acid 5 perature (about 70 F.). The electrolyte used to produce after sealing, were s'nbstantially srnudge free. As to all the original anodic oxide coating in each instance, the specimens there 'was no indication that the acid treatment thickness of the anodic oxide coating in each instance, the step had adversely affected the sealed condition of the loss in weight by the acid dissolution test identified herecoatings. All specimens in this example which had some inabove, and the condition of the anodic oxide coating smudge remaining andwere not substantially smudge free 10 surface both before and after the acid immersion step after the acid treatment step were capable of being wet are reported in the following table for each specimen wiped to a substantially smudge free condition. identified.

TABLE II Weighitloss Py aei sso u- Anodie oxide tion after coating thlck- Condition of anodic oxide Condition of anodic oxide HNOstreatness before coating after sealing and coating after HNO treatment (mg./ Aluminum base alloy Electrolyte sealing (ID-11S) before HNOa treatment menl; sq. in.)

SuJiophthalic/sulfurlc 0. 4 A 0. 5

1'. 22 01 7 0.95 1.1 0. 80 .-do Some smudge removed 0. 8 1.20 ..do Smudge-free 0.5

l The smudge remaining on this specimen was removed by wet Wiping.

Exampk 6 While the invention has been described in terms of preferred embodiments, the claims appended hereto are intended to encompass all embodiments which fall within the spirit of the invention.

Having thus described my invention and certain embodiments thereof, I claim:

1. A process for improving an anodic oxide coating on aluminum comprising (1) sealing an anodic oxide coating on aluminum which has not been organic dye colored while forming a chalky adherent but removable sealing smudge sub- This example illustrates that when the original anodic oxide coating before the sealing step of the invention is at least about 0.7 mil in thickness, the preferred use of a concentration of hydrolyzable metallic salt of at least about 1 g./l. in the sealing step of the invention facilitates removal of smudge in the acid treatment step. Two specimens of Aluminum Association Alloy No. 1100 clad with Aluminum Association Alloy No. 1100 having an anodic oxide coating of about 0.8 mil prodluced fay treatment ii! at sulfo hthalic acid/sulfuric acid eectro yte were seae for 1% minutes, one with 2 g./l. and the other with 5 g./l. i covermg 0 anodlc Oxlde coatlng y of boiling aqueous nickel acetate. Acidity was adjusted ii; f wlth a q eous hydrolyzable during the sealing step to a pH of 6. At this point (after 0 1 a A 0 a concentration of t east about sealing) the specimen sealed with 2 g./l. nickel acetate exhibited a white, chalky smudge on the surface of the removmg seaflng g m sa d coatsealed anodic oxide coating thereon, and the specimen a to g Coatmg a capablllty for sealed with 5 g./l. nickel acetate exhibited at heavy White, f ere 9 of sequenfly PP lacquer chalky smudge on the sealed anodic oxide coating. All I eatvlng Sal r lg l l y treatlng salne four specimens were then immersed for 3 minutes in 41 qfi t i i or er at ast one minweight percent nitric acid at room temperature (about a eas one aqueous ral flCld.

2. The process of claim 1 wherein when id anod' r substantial] smud e sa 1c if; a lllfilisihfifiiilliiiri ifid y g oxlde coatmg a thwkness of a least out 0.7 mil, the concentration of the hydrolyzable metallic salt is at Example 7 55 least about 1 g./l. This example illustrates several representative aluminum The Process 0 1 wherein the hot aqueous ybase alloys which may be treated according to the invend ro lyzable metalllc salt 15 Selected fr m the group contion X panels of the following alloys having anodic slstmg of sulfate and acetates of aluminum, nickel, cobalt,

h zinc copper lead and alkali metals. oxide coatings thereon were treated according to the i process of the invention. 4. The process of claim 1 wherein the hot aqueous I hydrolyzable metallic salt is selectedfrom the group con- AlloyhiTAlllmm gn ?Z Z J ZI I O I%Y 1100 clad sisting of cobalt acetate and nickel acetate.

Wl umlnurn SSOCIH y 5. The process of claim 1 wherein the mineral acid Allqit h g-f i gzi tf g lls g'o g g clad concentration is at least about 24% by weight. AIR )2, c g i l l llii gum z? z 1 5086 clad 6. The process of claim 1 wherein the mineral acid with Aluminum Association Alloy No. 5086 compnses mtnc acld' Alloy DAluminurn Association Alloy No. 3003 Clad T proces? of.clalm 1 Wherem the mmeral and with Aluminum Alloy containing in percent by weight compnses Sulfunc 5 Si, 0.30 Fe, 0.09 Cu and 0.55 Mn, balance substan- A Pmess for mPmVmg an X1de mung tially A1 aluminum comprising I Alloy E-Aluminum Association Alloy No. 3003 Clad creating a sealed coating condltlorg whlle f a with Aluminum Alloy Containing in percent by weight chalky adherent t movable sealing smudge sub- 0.20 Si, 0.62 Fe, 0.07 Cu and 0.35 Mn, balance substantially covermg an anodic oxide coating which has stantially Al not been organic dye colored and is present on an 7 aluminum substrate by treating the coating with a hot aqueous hydrolyzable metallic salt of a concentration of at least about 0.1 g./ 1.,

(2) thereafter removing sealing smudge from said coating while leaving said coating in said sealed condition by treating same for at least one minute with at least one aqueous mineral acid, and

(3) subsequently applying a lacquer, paint or enamel which adheres to said coating.

References Cited UNITED STATES PATENTS 9/1972 Treiber 204--38 A 3,418,219 12/1968 Fahlbusch 20 35 N 15 8 3,264,158 8/1966 Howe 20435 N 3,098,018 7/1963 Kissen et al 204 35 N 3,016,293 1/1962 Cybr'iWsky et a1. 204-35 N 2,376,082 5/1945 Pullen 204-35 N OTHER REFERENCES Sealing of Anodic'Films on Al and its Alloys, by Spooner, Metal Finishing, January 1969, pp. 80-82.

TA-HSUNG TUNG, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. 'X.R. 204-38 A, E

NiTn STATES PATENT orrict EER'MMATE @QECHN Patent No. 3,7915940 Dated February 12, 1974 Inventor(s) Andrew Alexander It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 63 Delete "Representative of hydrolyzable metallic salt per liter of water."

(first occurrence) Column 2, line 60 After ".anodic" change "coaitng" to v coating, v a Column 5, line 5 After a "with" change "sulfuruic" to sulfuric-'==-. i

Signed and "eealed this 11th day of June 1971!.

(3mm Atteet:

1 Patents D M..FLETCHER,JR. Attesting Officer FQRM PC4050 (10459) 7 US'COMM-DC 80376-1 69 I a U.5. GOVERNMENT PR|NT|NG OFFICE: I969 -366"5l4,